The standard for Cellular Mobile Communication Network and various technologies outlined in this invention are provided by The Third Generation Partnership Project (3GPP) consortium.
FIG. 1a is a technology timeline indicating evolution of existing WCDMA specification to provide increased downlink throughput. Referring to FIG. 1a, there is shown various wireless technologies including General Packet Radio Service (GPRS) 100, Enhanced Data rates for GSM (Global System for Mobile communications) Evolution (EDGE) 102, the Universal Mobile Telecommunications System (UMTS) 104, and the High Speed Downlink Packet Access (HSDPA) 106.
The GPRS 100 and the EDGE 102 are two technologies utilized for enhancing the data throughput of present second generation (2G) system, for example, the GSM. The GSM technology supports data rates of up to 14.4 kilobits per second (Kbps), while the GPRS technology 100 introduced in 2001 supports data rates of up to 115 Kbps by allowing up to 8 time slots for data per time division multiplex access (TDMA) frame. The GSM technology, by contrast, allows only 1 time slot for data per TDMA frame. The EDGE technology 102, introduced in 2003, supports data rates of up to 384 Kbps. The EDGE technology 102 utilizes 8 phase shift keying (8-PSK) modulation for greater data rates than the GPRS technology 100. The GPRS 100 and EDGE 102 may be referred to as “2.5G” technologies.
The UMTS technology 104, introduced in 2003 with theoretical data rates as high as 2 megabits per second (Mbps), is an adaptation of the wideband code division multiple access (WCDMA) 3G system by GSM. One reason for the high data rates of the UMTS technology 104 stems from the WCDMA channel bandwidths of 5 MHz versus the 200 KHz channel bandwidths for GSM. The HSDPA technology 106 is an Internet protocol (IP) based service oriented for data communications, which adapts WCDMA to support data transfer rates of the order of 10 Mbps. The HSDPA technology 106 achieves higher data rates through a plurality of methods. For example, many transmission decisions may be made at the base station level, which is much closer to the user equipment as opposed to being made at a mobile switching center or office. These may include decisions about the scheduling of data to be transmitted, when data are to be retransmitted, and assessments about the quality of the transmission channel. The HSDPA technology 106 may also utilize variable coding rates in transmitted data. The HSDPA technology 106 also supports 16-level quadrature amplitude modulation (16-QAM) over a high-speed downlink shared channel (HS-DSCH), which permits a plurality of users to share an air interface channel.
A WCDMA base transceiver station (BTS) may transmit a signal that may be reflected and/or attenuated by various obstacles and surrounding objects while propagating through the media. As a result, various copies of the transmitted signal, at various power levels, may be received at the mobile terminal with various time offsets introduced. The plurality of signals received by the mobile terminal may be referred to as multipath signals (multipaths). The propagation media may be referred to as the RF channel. Considerable effort is typically invested in recovering the received signal from distortion, interference and noise that may have been introduced while the received signal propagated through the RF channel. The RF channel may be characterized by its RF bandwidth and whether it consists of a single signal path (path), referred to as flat fading channel, or multipath signals, referred to as selective fading channel.
Mathematically, the effect on the signal at the mobile terminal antenna, at a time, may be expressed as a complex weight that denotes gain and RF phase. The weight may vary in time as the mobile terminal user moves from one location to another. Furthermore, the path energy, which may be determined by taking the absolute value of the square of the weight, may increase or decrease in a relatively short period of time. Since the received signal may be a superposition of the multiple signals from all paths, the impact of the RF channel may be characterized as a time-variant channel where the aggregate of the weights, which may be time variant channel impulse responses, may be known by their statistical properties. To recover the transmitted information from the received signal that may have been affected by the RF channel, a rake receiver is typically used.
A rake receiver may determine an estimate for each of the detected paths by correlating, or descrambling, the input signal with a Gold code (GC) that may be timely synchronized with a detected path. Thus the rake receiver may be equipped with a bank of correlators where each correlator may timely track the time position of a detected path. The rake receiver may weight a path by a complex conjugate of the path estimate. The individual path estimates may be timely compensated for their delay and combined. The combining operation, which may be a summation of the weighted signals, is known in the art as maximum ration combining (MRC).
Within the context of a rake receiver terminology, a finger may be the apparatus that may be assigned to a detected path, tracks it timely, produces the path estimate and weights the path estimate by the conjugate of the path estimate. A plurality of fingers may be assigned to track and demodulate a plurality of multipath signals. The output of the fingers may then be combined and further demodulated and decoded.
A considerable part of receiver design may involve managing the rake receiver fingers. A functional block in the rake receiver known as a “searcher” may locate new multipath signals and subsequently allocate a finger to each new multipath signal. The searcher may detect a signal path based on the amount of energy contained in a signal, identify that signal path if it carries user's data, and subsequently monitor the detected signal path. Once the detected signal energy in a path is above a given threshold, a finger in the rake receiver may be assigned to the path and the signal energy level may be constantly monitored.
However, partitioning a received signal into several fingers, each of which may process and exploit energy in a single path, may have limitations. For example, multipath signals may rarely be characterized by distinct, discrete times of arrival. As a result, the rake receiver may be inefficient at exploiting the power in the received signals. In addition, managing the fingers may incur high processing overhead. The total amount of time required to identify a path, assign a finger, and exploit the signal energy may account for 20-30% of the life span of a path.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.